This invention is generally directed to liquid developer compositions and, more specifically, to liquid developers containing BAB quaternary block copolymer charge directors. More specifically, the charge directors of the present invention are derived from alkyl or proton quaternized 2-ethylhexyl methacrylate-co-2-dimethylaminoethyl methacrylate, (EHMA/DMAEMA) BAB or B(AA')B triblock copolymers, and specifically derived from the alkylation or protonation of poly-2-ethylhexyl methacrylate-co-2-N,N-dimethylaminoethyl methacrylate (EHMA-DMAEMA) with alkylating agents or Bronsted acids, and wherein the resulting developers possess high charging levels, excellent more rapid initial charging rates, excellent conductivity characteristics, and wherein the mobility of developers prepared from these charge directors is about as effective as that of the corresponding AB block polymers, it is believed, since the BAB polymer charge directors of the present invention are soluble in the liquid of the developer. The BAB charge directors of the present invention are preferably comprised of two nonpolar B block components and one polar ammonium A or (AA') block component.
The developers of the present invention can be selected for a number of known imaging and printing systems, such as xerographic processes, wherein latent images are rendered visible with the liquid developer illustrated herein. The image quality, solid area coverage and resolution for developed images usually require sufficient toner particle electrophoretic mobility. The mobility for effective image development is primarily dependent on the imaging system used. The electrophoretic mobility is primarily directly proportional to the charge on the toner particles and inversely proportional to the viscosity of the liquid developer fluid. A 10 to 30 percent increase in fluid viscosity caused, for instance, by a 5.degree. C. to 15.degree. C. decrease in temperature could result in a decrease in image quality, poor image development and background development, for example, because of a 5 percent to 23 percent decrease in electrophoretic mobility. Insufficient particle charge can also result in poor transfer of the toner to paper or other final substrates. Poor or unacceptable transfer can result in, for example, poor solid area coverage if insufficient toner is transferred to the final substrate and can also lead to image defects such as smears and hollowed fine features. To overcome or minimize such problems, the liquid toners of the present invention were arrived at after substantial research efforts, and which toners result in, for example, sufficient particle charge for transfer and maintain the mobility within the desired range of the particular imaging system employed. Advantages associated with the present invention include a high developer particle charge over a range of low to medium conductivities; and further increasing the desired negative charge on the developer particles and in embodiments providing a charge director that is superior to similar charge directors like tetraalkyl quaternary ammonium block copolymers, lecithin, and metal salts of petroleum fractions. Some of the aforementioned additives like lecithin contain impurities which can have an adverse effect on their intended function. The superior charge can result in improved image development and superior image transfer. The low to medium conductivity of the dispersions obtained with the present invention improve the developability of the liquid toner dispersion as the high concentration of mobile ions in high conductivity liquid dispersions compete with the toner particles for the latent electrostatic image in the xerographic process. The high concentration of mobile ions, reduced with the present invention, can also disrupt other steps in the xerographic printing process such as the electrostatic transfer of the image from the image bearing member to a substrate. In a number of applications of the xerographic printing process, a subsequent electrostatic image is applied to the image bearing member over a previously developed image. In this process, often referred to as an image-on-image process, a high concentration of mobile ions in the first image would distort the electrostatic latent image being developed in the subsequent development. The desired low to medium conductivity for any given electrostatic printing process will depend on specific features of the printing system and it is, therefore, desirable to have a liquid toner dispersion with an adjustable low conductivity which provides high particle charges over a range of low to medium conductivities.
U.S. Pat. No. 5,019,477, the disclosure of which is totally incorporated herein by reference, discloses a liquid electrostatic developer comprising a nonpolar liquid, thermoplastic resin particles, and a charge director. The ionic or zwitterionic charge directors may include both negative charge directors, such as lecithin, oil-soluble petroleum sulfonate and alkyl succinimide, and positive charge directors such as cobalt and iron naphthanates. The thermoplastic resin particles can comprise a mixture of (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1 to 20 weight percent of the copolymer; and (2) a random copolymer of (iii) selected from the group consisting of vinyl toluene and styrene, and (iv) selected from the group consisting of butadiene and acrylate.
U.S. Pat. No. 5,030,535 discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. The liquid developers are prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to about 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture so that the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles, may be selected from known thermoplastics, including fluoropolymers.
U.S. Pat. No. 5,026,621 discloses a toner for electrophotography which comprises as main components a coloring component and a binder resin which is a block copolymer comprising a functional segment (A) consisting of at least one of a fluoroalkylacryl ester block unit or a fluoroalkyl methacryl ester block unit, and a compatible segment (B) consisting of a fluorine-free vinyl or olefin monomer block unit. The functional segment of block copolymer is oriented to the surface of the block polymer, and the compatible segment thereof is oriented to be compatible with other resins and a coloring agent contained in the toner whereby the toner is provided with both liquid repelling and solvent soluble properties.
Moreover, in U.S. Pat. No. 4,707,429 there are illustrated, for example, liquid developers with an aluminum stearate charge additive. Liquid developers with charge directors are also illustrated in U.S. Pat. No. 5,045,425.
The disclosures of each of the U.S. patents mentioned herein are totally incorporated herein by reference.
In U.S. Pat. No. 5,364,729, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for forming images which comprises (a) generating an electrostatic latent image; (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle with a melting point of at least about 25.degree. C., which developer has a melting point of at least about 25.degree. C., the contact occurring while the developer is maintained at a temperature at or above its melting point, the developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm at the temperature maintained while the developer is in contact with the latent image; and (c) cooling the developed image to a temperature below its melting point subsequent to development.
In U.S. Pat. No. 5,407,775, the disclosure of which is totally incorporated herein by reference, there is illustrated a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director comprised of a zwitterionic quaternary ammonium block copolymer wherein both cationic and anionic sites contained therein are covalently bonded within the same polar repeat unit in the quaternary ammonium block copolymer.
In U.S. Pat. No. 5,549,007, the disclosure of which is totally incorporated herein by reference, there is illustrated a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director comprised of an ionic or zwitterionic quaternary ammonium block copolymer ammonium block copolymer, and wherein the number average molecular weight thereof of said charge director is from about 70,000 to about 200,000.
In U.S. Pat. No. 5,484,679, the disclosure of which is totally incorporated herein by reference, there are illustrated liquid developers containing block polymer negative charge directors comprised of a total of at least three blocks, ammonium A blocks and nonpolar B blocks in various combinations, and more specifically, triblock copolymers of the formula A--B--A wherein the polar A block is an ammonium containing segment and B is a nonpolar block segment which, for example, provides for charge director solubility in the liquid ink fluid like ISOPAR.TM., and wherein the A blocks have a number average molecular weight range of from about 200 to about 120,000; the B blocks have a number average molecular weight range of from about 2,000 to 190,000; the ratio of M.sub.w to M.sub.n is 1 to 5; and the total number average molecular weight of the ABA copolymer is, for example, from about 6,200 to about 300,000, and preferably about 200,000.
In U.S. Statutory Invention Registration No. H1483, U.S. Pat. No. 5,306,591, U.S. Pat. No. 5,308,731, the disclosures of which are totally incorporated herein by reference, there is illustrated the following: a liquid developer comprised of a certain nonpolar liquid, thermoplastic resin particles, a nonpolar liquid soluble ionic or zwitterionic charge director, and a charge adjuvant comprised of an aluminum hydroxycarboxylic acid, or mixtures thereof; U.S. Pat. No. 4,306,591 discloses a liquid developer comprised of thermoplastic resin particles, a charge director, and a charge adjuvant comprised of an imine bisquinone; and U.S. Pat. No. 5,308,731 discloses a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director, and a charge adjuvant comprised of a metal hydroxycarboxylic acid.
Illustrated in U.S. Pat. No. 5,409,796 is a positively charged liquid developer comprised of thermoplastic resin particles, optional pigment, a charge director, and a charge adjuvant comprised of a polymer of an alkene and unsaturated acid derivative; and wherein the acid derivative contains pendant ammonium groups, and wherein the charge adjuvant is associated with or combined with said resin and said optional pigment; and in U.S. Pat. No. 5,411,834 is a negatively charged liquid developer comprised of thermoplastic resin particles, optional pigment, a charge director, and an insoluble charge adjuvant comprised of a copolymer of an alkene and an unsaturated acid derivative, and wherein the acid derivative contains pendant fluoroalkyl or pendant fluoroaryl groups, and wherein the charge adjuvant is associated with or combined with said resin and said optional pigment.